PVC theremin diamond speaker project

What he said....(Thierry and Peter)

This topic came up a month or so ago elsewhere, and this is what I wrote to one person who was confused by these issues.

"Loudspeakers can be used without a baffle, but they will quickly lose power at lower frequencies from cancellation. If you do not need to rattle the windows, this is not always a problem.

The simple pole-mounted diamond speaker is an "open baffle". Some audiophiles prefer open baffle loudspeakers for their clarity (uncolored by an enclosure) and "presence". The size of the baffle will determine at what frequency cancellation will become an issue. The speakers (drivers) used must have a higher self-dampening quality (electrical and mechanical 'Q' factor).

From the diagram someone provided once, the diamond speakers produced by Floyd Engles apparently had some sort of enclosure around the back of the speaker. This would allow better bass response with a smaller baffle. Some sound reinforcement companies, such as Atlas Sound, do sell "back boxes" for speakers up to 12". These are for (acoustical) support and dampening of ceiling and wall-mounted speakers.

Since they do not play down to the lowest frequencies, many guitar amps have an open-backed enclosure, which is simply a folded open baffle. This is also one way to get better bass response for an open-backed diamond speaker.

Acoustic suspension (closed) speaker cabinets are basically an "infinite baffle". They also provide some mechanical dampening of the speaker cone to help keep the sound "tight". They often contain some sound absorbing material as well. Their size and volume must be "tuned" to the speakers used and, for large speakers, the cabinet can become quite large. They can support very deep bass, but are less efficient and require more power.

The bass reflex (ported) enclosure is basically an acoustic suspension (infinite baffle) cabinet with a hole in it. The benefit is they are more efficient (require less power) and the cabinet is generally smaller. The port is tuned to the bass driver used to extend bass response, but at some point the "hole" will cause the cabinet to become unloaded and the bass response will fall off drastically. For large speakers, this is generally such a low frequency as to be a non issue if properly designed."

Designing speaker enclosures is both an art and a science, but this is far more important for speakers designed to reproduce all instruments at all frequencies. (Music reproduction rather than an instrument speaker.)

It will also add considerably to the size and weight of your speaker. This is something to consider when you're trying to balance it on the top of a tall pole.

A simple open baffle makes things much easier as well as imparting a more "airy" quality. A simple open baffle of 24 inches square (61cm) should give you good support to the lower viola/tenor range.

Achieving full support to the bottom of the cello range will take a baffle twice as large; about 50 inches square (122cm). In this case, an enclosed back (or open box) would probably be desirable. Then you get back to the size/weight issues.

Cool! Thanks for the technical info. I will keep this in mind when designing my diamond speakers in the future. :)

Remember, the earth shattering bass we've all come to know was not widely available in consumer electronics until around the 1970's. The development of foam surrounds and high excursion drivers made it possible to get rich, deep bass from relatively small enclosures.

Prior to that, low excursion speakers in relatively flimsy cabinets were the rule. The back of the enclosures were generally a flimsy piece of masonite, sometimes with an array of slots or holes. If the electronics were housed in the same cabinet, the masonite had an array of slots or holes, mainly for ventilation.

The early console radios, including the RCA-106 loudspeaker, had no real back at all; perhaps only a very thin dust cover (cloth).

By the way, as mentioned, I've got to replace the existing speaker. It's cone's going in the low-mid range. Very likely the result of letting 400 students try the theremin back on 30 Oct.

It's a cheap speaker anyway, but now it's got to go before I finnish the speaker.

It's a 20 watt speaker, but it's from a 20 watt amp, and they don't give any info about resistance, ohms, ect, just 20 watts.

Question: Is it safe to put a 25 watt speaker on an amp made for a 20 watt speaker?

Sure it is. In fact it gives you a slightly higher safety factor.

Speaker wattage ratings are not an exact science and are subject to some interpretation. It's likely they understate their power handling ability slightly for an extra cushion of safety (for the speaker).

The thing that destroys speakers (especially high frequency drivers) is not necessarily power, but DISTORTION! That is a function of the quality of the amp output, not the speaker power rating.

More often than not, this is caused by people turning an underpowered amp up too high. Just because the knob on an amp goes up to 11, doesn't mean you should turn it up that high!

Doing it on a guitar amp is one thing (and sometimes desirable). Doing it on your home stereo is the fastest way to destroy your speakers. A good rule of thumb is not to turn it more than about 2/3 of the way up and even that's pushing it (IMO).

If you need that much volume, you need a bigger amp. The purpose of high power amps is to provide HEADROOM so that they can play transients (sharp, percussive sounds) with minimal distortion.

A speaker with a power rating that's too low and an amp that puts out too much distortion is a recipe for disaster.

The only problem you may have, even though it is unlikely, is if the 25 watt speaker is only 2 ohms and your amplifier requires at least 4 or 8 ohms. In this case, you will likely blow the amp.

Thanks. This helps a lot.

There are just a few major failure modes for speakers.

1. Burnt voice coils ... this happens under several possible conditions:
a. The most common is too much power fed into it, causing the enamel on the wire to heat up. Which in turn causes the voice coil to get stuck and fry.
b. The next most common is similar, but the voice coil is just rubbing the pole piece and buzzing.

2. Tears in the cone. Usually induced by an "oops, I slipped" :)

3. If the speaker has a foam surround, often those will rot.

If it is a speaker you paid a healthy sum for, or one that you really liked, you can usually find someone in your area that does speaker reconing. That is much cheaper than replacing the speaker.

When they recone it, they usually replace the voice coil and set it up properly so that it will not rub.

Don

PS...

Stereo or surround speakers for music reproduction are generally designed for clean, flat response over the entire frequency spectrum. Of course, this can be spread across one, two, three, or four drivers.

Musical instrument speakers are generally designed for ruggedness, power, and "tone" over a limited frequency spectrum.

Theremins produce can produce a fairly wide frequency range, depending on the composition. Their output is monophonic and relatively pure, unless distortion is added intentionally. They produce virtually no transients.

For a monitor just a few feet behind your head, such as a diamond speaker, a high power amp is not necessary or desirable.

If you want to shake the rafters, you'll need a PA speaker or a subwoofer in a proper enclosure with an amp of sufficient power to drive it properly. The bass frequencies require the greatest power.

Jeff. Thanks for that info.

My goal is not to shake the rafters, or run distortion, but to have a little richness of a cello in the lows, while keeping the delicate vocal like qualities Coalport likes.

Sounds like my initial decision to use low end 20 to 25 watt amps was a good one afterall.

I have noticed that cello richness once I mounted the speaker on the soundboard, and a bit more than I wanted with the cardboard shell on the back. So, it looks like non-baffled is right for me. I've been using non-baffled speakers ever since I started mounting the speaker behind my head.

I just need to make certain to match replacement speakers to the original stock speakers.

I don't personally know anyone in the area who recones, but I've heard of it being done here.

As cheap as these little speakers are, I don't know if it's worth reconing, or just replacing the speaker. I've priced similar 20 to 25 watt speakers at around 30 to 40 dollar.

Thanks to everyone for your input. :)

You're welcome. I've offered a little more info than you've asked for to benefit those who may not be aware of these issues.

One of the specifications used to rate a speaker is the sensitivity or efficiency. This is described by the standand "1 watt at 1 meter" (1w/1m).

For most decent speakers, the measured value lies somewhere between 85dB and 95dB. So, at only 1 watt of power, about 90dB at three feet away would be considered quite loud by most people.

To make it seem twice as loud, or about 100 dB (+10dB), would require about ten times the power, or 100 watts. To make it twice as loud again, say 120dB (which would be extremely hazardous to your hearing), would take ten times the power again or about 1000 watts.

Those numbers are for "average" sound pressure levels. Transients can go much higher than the average for short periods of time.